This invention relates to the production of chlorothioformates by the reaction of a mercaptan with phosgene in the presence of an activated carbon catalyst, ##STR1## In this invention, R is alkyl, lower cycloalkyl-methyl, lower cycloalkyl, lower alkenyl, phenyl, chloro-substituted phenyl, benzyl or chloro-substituted alkyl in which the chloro substituent is situated at least as far as the .gamma. (gamma) carbon atom, with respect to the sulfur atom. By the term "alkyl" or "chloro-substituted alkyl" is meant such groups having from 1 to 15, preferably from 1 to 10, and most preferably from 1 to 6, carbon atoms, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec.- butyl, isobytyl, n-pentyl, neopentyl, n-hexyl, neohexyl, n-heptyl, n-octyl, n-decyl, n-dodecyl and n-tetradecyl. By "lower alkenyl" is meant such groups having from 2 to 5 carbon atoms and at least one olefinic bond. By "lower cycloalkyl" is meant cycloaliphatic groups having from 3 to 7 carbon atoms, such as cyclopropyl and cyclohexyl. The term "lower cycloalkyl-methyl" includes groups having from 3 to 7 carbon atoms in the cyclo alkyl portion, such as cyclopropylmethyl and cyclopentylmethyl. The term "chlorophenyl" includes both mono- and polychlorinated phenyl rings in which the chlorine atom or atoms may be variously substituted.
In a preferred embodiment of this process, R is alkyl, lower cycloalkyl, lower cycloalkyl-methyl, benzyl, phenyl or chloro-substituted phenyl. Preferred embodiments for the various possibilities for R are: for alkyl- such groups having from 1 to 6 carbon atoms, particularly ethyl, n-propyl, isopropyl, n-butyl, sec.-butyl, n-pentyl and neopentyl; for lower cycloalkylcyclobutyl; for lower cycloalkyl-methyl-cyclopropylmethyl and cyclopentylmethyl; for the lower alkenyl- allyl; for chloro-substituted-phenyl-, p-chlorophenyl; for the haloalkyls- 3-chloropropyl.
Such chlorothioformates are useful intermedites for the production of herbicidally effective thiocarbamates and similar compounds. This reaction between mercaptans and phosgene to produce chlorothioformates is described in U.S. Pat. No. 3,165,544 of Harry Tilles, which discloses the conduct of this process in laboratory size equipment. It is pointed out that reaction temperatures should be maintained as low as possible, consonant with reasonable reaction rates since at high temperatures a disulfide by-product begins to form in significant amounts. Maximum temperatures are suggested for this reaction of between about 70.degree. and 140.degree. C.
One process which has been utilized for commercial scale production of lower alkyl chlorothioformates by this reaction employs two catalytic beds of activated carbon arranged in series. The first bed is preferably contained in tubes of a multi-tube reactor; the second is in the form of a packed bed reactor containing a single catalyst bed. The first reactor is operated as a continuous liquid phase reactor; more specifically as an upflow tubular catalytic reactor, with starting materials introduced at the bottom and products removed from the upper portion. The partially reacted mixture is then introduced into the top of the second reactor, which functions as a trickle-flow (downflow) packed bed. That is, the second reactor is operated in the continuous gas phase since gaseous hydrogen chloride product is continuously passing through the bed. Reaction products are removed from the lower portion of the second reactor and passed to downstream apparatus for separating the chlorothioformate. Operation of this process for production of ethyl chlorothioformate, however, has been found to produce this product in a purity of only between about 91 and about 95%. The major impurity is diethyl disulfide, present in about 3-7% concentration, with most of the remaining impurities being diethyl dithiocarbonate. When used to produce n-propyl chlorothioformate, the amount of disulfide by-product ranged from 1.5-13.7% and averaged just under 5% and the chlorothioformate purity averaged about 93%.
It is an object of the present invention to provide an improved process for the production of chlorothioformates by reaction of a mercaptan and phosgene in the presence of an activated carbon catalyst.
A further objective of the present invention is to provide such a process with minimization of a disulfide by-product.
A third objective of the present invention is to provide such a process with enhanced production capacity.
Yet another object of the present invention is to provide such a process having good temperature control in the reactors.
A still further objective of the present invention is to provide such a process having a good conversion of the mercaptan to chlorothioformate.